This phase I trial for safety and efficacy of a novel drug against human immunodeficiency virus (HIV) requires adequate representation across the pediatric age spectrum. Escalation in dosage requires completion of all age strata. Very few HIV-infected children have been identified under age 2 years, reflecting the success of AZT prophylaxis against maternal to infant transmission. Escalation of ACTG 310 was stymied because of lack of enrollment of younger children. An intensive effort at our institution resulted in enrollment of the entire national age < 2 yrs cohort, establishing safety and bioavailability, and promoting subsequent escalation in dosage of the trial. We anticipate providing additional representation in this crucial age range at the higher drug dosage. Successful completion of this trial will lead to an efficacy trial of adefovir dipivoxil in children. Adefovir is the first drug against HIV targeted for early trials in children concurrent with trials in adults. Early trials in children are necessary to address the severe shortage of anti-HIV drugs in children relative to adults (3 vs. 10 drugs available in 1996). Adefovir is currently in early trials in adults as part of new generation "cocktails" in combination with protease inhibitors. Pediatric trials should follow shortly. Adefovir is likely to improve pediatric HIV care, providing more options for "pediatric cocktails" against HIV. The drug has promising activity against cytomegalovirus, an HIV co-pathogen in children, making it doubly attractive.